The recovery of energy resources (e.g., petroleum and other fluid hydrocarbons, or geothermal resources) from a deposit beneath the ocean floor conventionally involves a drilling operation to reach the deposit, and a production operation to bring the resources from the undersea deposit to the ocean surface. An offshore drilling and production operation generally requires a buoyant structure such as a vessel or other floating structure on the ocean surface, stationary equipment positioned on the ocean floor, and a conduit structure extending between the buoyant structure and the stationary equipment.
In a drilling operation, a conduit structure called a "drilling riser" extends between the buoyant structure on the ocean surface and the stationary equipment at the drilling site on the ocean floor. The stationary equipment for a drilling operation usually includes a blowout preventor (customarily called a "BOP"), which is lowered along with the drilling riser from the buoyant structure to the drilling site. Coupling of the drilling riser to the BOP is conventionally accomplished using a remotely operated connector. The drilling riser encloses a pipe (called a "drill string"), which is attached to a bit for drilling a hole in the ocean floor to the energy resource deposit. The drilling riser and the drill string thus form concentric conduits. A viscous fluid material (called "drill mud") for controlling down-hole pressure and for washing away drilling debris is passed down the drill string to where the drill bit cuts into the ocean floor, and the resulting mixture of drill mud and debris (called "return mud") is forced up the annular region between the drill string and the drilling riser to the buoyant structure on the ocean surface. Typically, the drilling riser also supports external tubular hydraulic conduits and/or electrical cabling for powering and/or controlling the BOP and the remotely operated connector.
In a production operation, a conduit structure called a "production riser" extends between the buoyant structure on the ocean surface and the stationary equipment at the production site (i.e., the well) on the ocean floor. The stationary equipment for a production operation usually includes a retrievable assembly of valves and piping (called a "subsea manifold") for connecting flow lines leading from the well to one or more transport conduits enclosed within the production riser. Depending on the particular surface arrangements for storage and loading of the energy resource recovered from the undersea deposit, the production riser may also enclose tubes through which the energy resource can be pumped back to storage containers on the ocean floor. Coupling of the production riser to the subsea manifold is conventionally accomplished using a remotely operated connector. The production riser can also be used to enclose or support tubular hydraulic conduits and/or electric cabling for powering and/or controlling the subsea manifold, the remotely operated connector and other production equipment.
In installing stationary equipment on the ocean floor for a drilling or production operation, proper orientation and positioning of the equipment is usually of major importance. A customary technique for achieving proper orientation and positioning of the stationary equipment utilizes two or more guidelines extending from the buoyant structure to the site at which the equipment is to be installed on the ocean floor. The guidelines are kept very taut in the vertical direction by upwardly directed tensioning forces. The equipment is then secured to the guidelines so that, as the equipment is being lowered to the ocean floor, downward vertical motion parallel to the guidelines is the only motion possible for the equipment. The guidelines prevent rotation and lateral displacement of the equipment away from the required orientation for proper installation.
Usually, a marine riser is run downward from the floating structure to the ocean floor is discrete cylindrical pipe segments. The pipe segments are attached to each other, one after the other, until the complete riser is formed.
In operation, marine risers and guidelines must be kept under substantially constant upward tension. Drilling and production risers, if not kept taut by a constant upward tension, would be damaged or destroyed by compressional loading that would cause the risers to buckle and bend. Similarly, guidelines, if not kept taut by a constant upward tension, would become slack and be incapable of properly orienting and positioning the equipment being lowered to the ocean floor.
Local conditions (e.g., waves, tides, winds, surface and subsurface currents, and other phenomena occurring in a marine environment) cause a buoyant object floating on the surface of the ocean or other body of water to undergo a variety of motions. In particular, a vessel or other type of floating structure used in offshore drilling and/or production operations heaves and sways on the ocean surface in response to such local maritime conditions. The magnitude of the heaving and swaying of the floating structure is dependent upon the hull form response characteristics of the floating structure, as well as upon the type of mooring or positioning system used to maintain station over the drilling or production site, and upon changes in the draft of the floating structure. Thus, for example, a typical shipshaped hull in a conventional mooring arrangement would undergo more pronounced heaving and swaying than a semi-submersible floating platform of the tension leg type. Nevertheless, regardless of any heaving and swaying motions of the floating structure on the ocean surface, the marine risers and guidelines used in an offshore drilling and/or production operation must be maintained at a substantially constant upward tension.
The required upward tensioning force on a marine riser is ordinarily applied at or near the upper end of the riser to overcome the compressional loading due to gravity, which is proportional to the weight (and therefore to the length) of the riser. For drilling and production operations in very deep water, it may also be advantageous in certain applications to apply buoyant forces along at least part of the submerged length of a marine riser.
Techniques used in the prior art for maintaining constant upward tension on marine risers and guidelines have generally employed gas-over-oil hydraulic cylinders and accumulators for applying tension through complex pulley systems requiring cables and sheaves or chains and sprockets. In a typical marine riser tensioning system of the prior art, sheaves were attached to the floating structure on the ocean surface, and tensioning cables were run upward from the riser and over the sheaves (one cable for each sheave) so that the riser was suspended from the sheaves by the tensioning cables. Each tensioning cable was connected to a hydraulic cylinder, which allowed the cable to move in either direction over its corresponding sheave as necessary to maintain a constant upward tension on the riser. The hydraulic cylinders were typically actuated by accumulators that were sized to permit only small variations in tension on the riser over a wide range of vertical position changes for the floating structure relative to the riser.
It was common in the prior art for marine riser tensioning cables to be attached to lugs on a bearing ring fitted over and affixed to the upper end of the riser. The cables applied an upward tensioning force to the riser via the bearing ring, which was free to rotate around the riser so as to permit rotational motion of the riser relative to the floating structure. The bearing ring generally contained lubrication means to minimize the transfer of torque between the floating structure and the riser. Suspension of a marine riser by tensioning cables accommodated changes in position of the floating structure relative to the riser without imposing bending moments on the riser. However, such prior art tensioning systems generally had limited operational life, and required frequent and substantial maintenance efforts. In particular, marine riser tensioning systems of the prior art frequently sustained tensioning cable failures due to metal fatigue, and encountered leakage problems associated with hydraulic and pneumatic seals.
Techniques used in the prior art for maintaining constant upward tension on guidelines generally included use of means similar to those described above for tensioning marine risers. Thus, for guidelines as well as for marine risers, the prior art provided no practical alternative to the use of cables or chains and hydraulic cylinders for applying upward tension.